If Muammar Gaddafi is listening to advice as he fights to save his regime, it is likely to be coming from his brother-in-law and chief enforcer, Abdullah Senussi.

Senussi, who is married to a sister of Gaddafi's wife, is considered to be the Libyan leader's most trusted aide. He has had a reputation for brutality since the mid-1970s and his name appeared as number two on an opposition list of wanted "war criminals" topped by Gaddafi.

"Senussi will support Gaddafi until he dies," said Noman Benotman, a former Libyan jihadi who knows Senussi well. "He's a brutally honest guy, not at all sophisticated. They all agree on using force to crush the uprising."

Senussi is being blamed for the killings in the eastern city of Benghazi over the last few days as well as recruiting the foreign mercenaries fighting in the service of the regime. Libyans hold him responsible for the notorious 1996 massacre of about 1,200 inmates at the Abu Salim prison.

In the early 1980s, when he was Gaddafi's head of internal security, many opponents of the regime were killed.

Described in recent years as head of military intelligence with the rank of general, it is unclear whether he still holds any formal position, but he is certainly part of Gaddafi's "ahl al-Khaimah" (people of the tent) – his very closest entourage.

"He is well known, and feared, throughout the country," said one exile. "He's very ruthless and very ambitious. Wherever the colonel goes, Senussi is there, too. Gaddafi trusts him with his life."

Senussi has kept a low profile in recent years, partly because he has been unable to travel abroad since being convicted in absentia in France in 1999 for his role in the 1989 bombing of a UTA passenger plane over Niger which killed 170 people.

At the time, he headed Libya's external security organisation, in which capacity he was said to have recruited Abdel-Basset al-Megrahi, the man convicted of the 1988 Lockerbie bombing. Like Megrahi, Senussi is a member of the powerful, and apparently still loyal, Megarha tribe, based in the Sebha area on the edge of the Sahara. He is also a cousin of Abdel-Salam Jalloud, one of Gaddafi's oldest comrades.

US embassy cables released by WikiLeaks describe him as a confidant of the leader who also makes "many of his medical arrangements". Like other senior Libyan figures Senussi has extensive private business interests. Libyan, Arab and western sources describe Senussi as a thuggish figure who would personally beat and abuse prisoners. "He is the eyes and ears of and Gaddafi and his enforcer," said one North African source who knows the regime well.

Sanussi was also said to have been behind an alleged Libyan intelligence plot to assassinate Crown Prince Abdullah of Saudi Arabia in late 2003.

Tunisians and Egyptians are fleeing Libya, many across its western land border, as the embattled regime of Muammar Gaddafi cracks down on protests. The EU is preparing to evacuate 10,000 stranded Europeans